PO15: Systematic Dwell Position Deviation of Elekta MR Compatible HDR Ring Applicators

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<h3>Purpose</h3> (1) To identify a systematic dwell position deviation of Elekta MR compatible HDR ring applicators due to its design and (2) to demonstrate the dosimetric impact of the deviation. <h3>Materials and Methods</h3> In these ring applicators, the source is designed to travel precisely along the central axis of the ring tube. However, the actual source positions deviate from the central axis due to the semi-rigid design of the wire carrying the source. To quantify the deviation, in-house QA devices were built using decommissioned ring applicators of three different diameters (26mm, 30mm, and 34mm). The ring applicators were modified by replacing their top caps with a 1 mm thick transparent plastic plate. This allowed proper visualization of both the vendor-provided wire QA markers and the actual Ir-192 source. The high-resolution camera of the Elekta Flexitron system was used to acquire images of the applicator and the source. The source was driven to dwell positions with a 5 mm increment to cover the full visible range of the ring. An image was acquired at each dwell position. This procedure was repeated 5 times for each ring size. Two observers independently measured the arc length traveled by the source from the first dwell position to other dwell positions. The deviation was calculated as the difference between the measured arc length and the intended dwell position (converted to arc length). <h3>Results</h3> The deviation was found to be a function of the arc distance from the first dwell position. It increases almost linearly with arc length, experiences a sharp increase before settling in a range between 5 mm and 6 mm. Up to 6 mm deviation was found for ring applicators of all three different diameters. Although there were small interobserver differences (mean = 0.1 mm, SD = 0.13 mm), the results were highly reproducible (SD = 0.09 mm). The dwell position deviations result in remarkable differences of the isodose curves surrounding the ring applicator. The measured deviations were applied to our library plan for cervical cancer using 30 mm ring and 60 mm tandem. The isodose lines in the plane of the ring before and after the offsets were applied are shown in figure 1(a) and (b), respectively. <h3>Conclusions</h3> Large systematic dwell positions deviations of the Elekta MR compatible HDR ring applicators has been quantified. These deviations may have significant dosimetric impact on HDR treatment. Therefore, they should be considered by the treatment planning system for more accurate dose computations.